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Light-year
A light-year, also light year or lightyear (symbol: ly) is a unit of length, equal to just under 10 kilometers (10 meters, 10 petameters or about 6 trillion miles). As defined by the (IAU), a light-year is the distance that in a in one . The light-year is often used to measure distances to stars and other distances on a scale, especially in non-specialist and publications. The preferred unit in is the parsec, because it can be more easily derived from, and compared with, observational data. The parsec is defined as the distance at which an object will appear to move one arcsecond of when the object moves one astronomical unit perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light-years. Numerical value One light-year is equal to: * exactly 9,460,730,472,580.8 km (about 9.5 Pm). * about 5,878,625,373,183.6 miles (about 6 trillion miles). * about 63,241.1 astronomical units. * about 0.306595 parsecs. * exactly 31,557,600 light-seconds. The figures above are based on a (not ) of exactly 365.25 days (each of exactly 86,400 SI seconds, totaling 31,557,600 seconds)IAU Recommendations concerning Units and a defined speed of light of 299,792,458 m/s, both included in the , used since 1984.Astronomical Constants page K6 of the . Other values Before 1984, the (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983.P. Kenneth Seidelmann, ed., Explanatory Supplement to the Astronomical Almanac (Mill Valey, California: University Science Books, 1992) 656. ISBN 0-935702-68-7 The product of 's mean tropical year of 31,556,925.9747 s and a speed of light of 299,792.5 km/s produced a light-year of (rounded to the seven in the speed of light) found in several modern sourcesSierra College, Basic ConstantsMarc Sauvage, Table of astronomical constantsRobert A. Braeunig, Basic Constants was probably derived from an old source such as 's 1973 Astrophysical Quantities reference work,C. W. Allen, Astrophysical Quantities (third edition, London: Athlone, 1973) 16. ISBN 0-485-11150-0 which was updated in 2000.Arthur N. Cox, ed., Allen's Astrophysical Quantities (fourth edition, New York: Springer-Valeg, 2000) 12. ISBN 0-387-98746-0 Other high-precision values are not derived from a coherent IAU system. A value of found in some modern sourcesNick Strobel, Astronomical Constants B Astronomical Constants is the product of a mean Gregorian year of 365.2425 days (31,556,952 s) and the defined speed of light (299,792,458 m/s). Another value, ,Thomas Szirtes, [http://books.google.com/books?id=8Fk__-TUdCEC&pg=PA60 Applied dimensional analysis and modeling] (New York: McGraw-Hill, 1997) 60.Sun, Moon, and Earth: Light-year is the product of the J1900.0 mean tropical year and the defined speed of light. History The first successful measurement of the distance to a star other than our Sun was made by in 1838. The star was , and he used a superlative designed by . The largest unit for measuring distances across space at that time was the Astronomical Unit (AU), equal to the radius of the Earth's orbit ;}} ).}} The use of this unit in trigonometric calculations based on 61 Cygni's of 0.314 arcseconds, gave the distance to the star as . Bessel realised that a much larger unit of measurement was needed to make the vast interstellar distances comprehensible. had stated in 1729 that light travelled 10,210 times faster than the Earth in its orbit. In 1769, a revealed the distance of the Earth from the Sun, and this, together with Bradley's figure, allowed the speed of light to be calculated as , very close to the modern value. Bessel used this speed to work out how far light would travel in a year, and announced that the distance to 61 Cygni was 10.3 light-years. This was the first appearance of the light-year as a measurement of distance, and, although modern astronomers prefer the parsec, it is popularly used to gauge the expanses of interstellar and intergalactic space. Distances in light-years Distances measured in fractions of a light-year (or in light-months) usually involve objects within a . Distances measured in light-years include distances between nearby s, such as those in the same or . One kilolight-year, abbreviated "kly", is one thousand light-years (about 307 parsecs). Kilolight-years are typically used to measure distances between parts of a . One megalight-year, abbreviated "Mly", is one million light-years (about 307 kiloparsecs). Megalight-years are typically used to measure distances between neighbouring galaxies and . One gigalight-year, abbreviation "Gly", is one billion (109) light-years—one of the largest distance measures used. Gigalight-years are typically used to measure distances to supergalactic structures, including s and the . Light-month The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures. The specifies the light month more precisely as 30 days of light travel time.Light-Travel Time and Distance by the Hayden Planetarium Accessed October 2010. See also * * Light-second * 1 petameter (examples of distances on the order of one light-year) * Orders of magnitude (length) * References